The token system that Bee-dubya-ell describes worked well on our visit to the States for the wonderful cajun music festival in Lafayette - http://www.festivalsacadiens.com/index1.html - and it applied to food and crafts as well as drinks.
The whole festival was free admission but people were asked not to bring their own food and picnics to the event so that the festival could pay its way from their cut of all the stall takings. It also meant that the stall holders paid according to the amount of business that they did rather than a flat fee.
The whole system seemed to work very well.